SYDNEY—Eight aircraft joined the search Sunday for missing Malaysia Airlines Flight 370 in the southern Indian Ocean, as Australian Prime Minister Tony Abbott said authorities had a number of credible leads, including satellite images of an object in the area from Beijing.

A Chinese defense agency said Saturday an analysis of satellite images showed a large object, 22 meters (72 feet) long and 13 meters wide, floating in the southern Indian Ocean. Mr. Abbott said the Chinese images were consistent with earlier satellite...